Magnetic-field induced gap and staggered susceptibility in the S=1/2 chain[PM center dot Cu(NO3)(2)center dot(H2O)(2)](n) (PM = pyrimidine)

Single-crystal magnetic susceptibility and specific heat studies of the one -dimensional copper complex [PM . Cu(NO3)(2)bullet>(*) over bar * (H2O)(2)] (n) (PM = pyrimidine) show that it behaves Like a uniform S = 1/2 antiferro magnetic Heisenberg chain, characterized by the exchange parameter J/k(B) = 36 K. Specific heat measurements in the applied magnetic field, however, r eveal the formation of a field-induced spin excitation gap, whose magnitude depends on the magnitude and direction of the field. This behaviour is inc onsistent with the ideal S = 1/2 Heisenberg chain. In the low-temperature r egion, a contribution to the susceptibility, approximately proportional to 1/T, is observed which varies strongly with the varying direction of the ma gnetic field. The field-induced gap and the 1/T contribution are largest fo r the same field direction. Previous observations of a field-induced gap in the related compounds copper benzoate and Yb4As3 have been explained by th e alternating g tensor and alternating Dzyaloshinkii-Moriya interaction, pr oducing an effective staggered magnetic field at the Cu and Yb ions. We app ly this model to [PM . Cu(NO3)(2)bullet>(*) over bar * (H2O)(2)](n) and obt ain a consistent quantitative explanation of the low-temperature susceptibi lity, the field-induced gap and their dependence on the magnetic-field dire ction.